1. Field of the Invention
The present invention relates generally to a high color rendering fluorescent lamp, and particularly concerns a high color rendering fluorescent lamp having a two-layered phosphor coating.
2. Description of the Prior Art
Hitherto, phosphors generally used in fluorescent lamps of general illumination use have been phosphors of antimony-manganese-activated calcium halophosphate. In this phosphor, depending on ratio of antimony and manganese concentrations added as additives, white light of various color temperatures is efficiently obtained. Therefore a highly efficient fluorescent lamp having light of broad spectral distribution is obtainable with a single compound of cheap expense as a starting material, and the phosphor is one of the most splendid from an industrial view point. However, in the fluorescent lamp using the abovementioned phosphor, though having high efficiency, the use of the lamp in a field of use wherein color rendering is regarded important has been limited, because they have poor color rendering by possessing a general color rendering index of about 55 to 75.
As a conventional fluorescent lamp having a high color rendering, for instance, the EDL (Extra De Lux) type fluorescent lamp, having two layers of phosphor coating is known. Therein, a first layer comprises mixture of several phosphors prepared so as to approximate the spectral distribution of a reference illuminant and a second, i.e., overriding layer functions to absorb the bright line spectra of mercury in a short wavelength part of the visible range, which are not desirable for attainment of high color rendering (Japanese Patent Examined Publication Sho No. 41-9868). In such conventional EDL type fluorescent lamp with the double-layered coating, as a result of forming a layer containing titanium oxide and yellow pigment immediately on the inner face of tube wall as the layer to absorb the spectral line of mercury in the short wavelength part of the visible range, loss to some extent of the lamp efficiency has been inevitable, and improvement of the efficiency has long been the object of improvement of the fluorescent lamp.
In recent years, improvement of efficiency of the high color rendering fluorescent lamp has been strenuously tried, and for the EDL type fluorescent lamp, which utilizes blue-green emitting phosphor comprising a phosphor of strontium-boron phosphate activated by a divalent europium, a phosphor of an alkaline earth halophosphate activated by divalent europium having the formula (Ba, Ca, Mg).sub.10 (PO.sub.4).sub.6 Cl.sub.2 :Eu, a phosphor of a strontium-magnesium aluminate activated by a divalent europium and a phosphor of strontium aluminate activated by divalent europium have been proposed as the material to suppress the spectral line of mercury, and an improvement in single layer coating to some extent has been achieved. In the above-mentioned single layer coating fluorescent lamp, however, the blue-green emitting phosphor functions to absorb all emission energy of even other phosphors, since the bluegreen emitting phosphor is distributed uniformly in the single-layered phosphor coating. Accordingly, the single-layered EDL type fluorescent lamp has a problem of insufficient output of all emission energy of the phosphors. Furthermore, since the blue-green emission phosphor has the above-mentioned absorption, the phosphor itself has a yellow-green body color, and therefore when coated in the fluorescent lamp in the single layer coating, even when the phosphor is used as a mixture with other phosphor, the appearance of the lamp face has yellow-green body color as far as the phosphor coating is a single layer.
The inventors carried out many detailed experiments in applications of many phosphors having absorption in the short wavelength part of visible range utilized together with other phosphor(s) for methods of taking out largest emission energy. An example of spectral reflection characteristics of blue-green emitting phosphors used in the experiments, that of a phosphor of strontium aluminate activated by a divalent europium (i.e. having the general formula Sr.sub.4-x Eu.sub.x Al.sub.14 O.sub.25) is shown in FIG. 1. As is obvious from FIG. 1, the blue-green emitting phosphor shows strong absorption in the visible short wavelength range and still has about 5 to 10% of continuous absorption in the side of longer wavelength. In FIG. 1, curve 1 is for Sr.sub.3.98 Eu.sub.0.02 Al.sub.14 O.sub.25, curve 2 is for Sr.sub.3.92 Eu.sub.0.08 Al.sub.14 O.sub.25, curve 3 is for Sr.sub.3.60 Eu.sub.0.40 Al.sub.14 O.sub.25 and curve 4 is for Sr.sub.3.20 Eu.sub.0.80 Al.sub.14 O.sub.25. As the concentration of europium increases, the absorptions in the visible short wavelength range increase and the continuous absorptions in longer wavelength part also increase. In order to utilize the absorptions in the visible short wavelength range more effectively and to decrease as small as possible the continuous absorption in the visible light at the longer wavelength part, the conventional method of providing on the inner face of the glass tube a phosphor layer comprising a phosphor having the absorption effect can not produce a sufficient result. The reason is that the emission energy of the fluorescent lamp necessarily pass through the underlying phosphor layer thereby causing absorption of a part at longer wavelength part of the visible light.
Furthermore, the inventors made experimental study on the examples of the fluorescent lamps with single-layered phosphor coating. In the single-layered coating type fluorescent lamp, the defect of the appearance of yellow-green body color can not be eliminated because the phosphor having the effect of the above-mentioned absorption is uniformly distributed in the single-layered phosphor coating.